Refractory coatings are required in certain illumination sources such as those containing metallic sodium or halogen compounds wherein it is desired to provide an envelope material which is relatively impervious to chemical attack.
Most glasses, containing SiO.sub.2, are attacked chemically by sodium, for example, rendering them useless as lamp envelopes. Conventionally, use is made of a plyglass in which the glass envelope is prepared with a B.sub.2 O.sub.3 -containing glass liner. Such glasses are relatively expensive and may be limited with respect to geometric forms which can be fabricated.
It is known to coat glass envelopes of lamp and cathode ray tubes with various materials which serve as protective coatings for the glass envelopes. U.S. Pat. No. 2,264,892 to Schaefer discloses coating the interior of cathode ray tubes with bentonite (Al.sub.2 O.sub.3.4SiO.sub.2.XH.sub.2 O) as an aqueous suspension. U.S. Pat. No. 3,067,356 to Ray discloses coating a fluorescent lamp envelope with an alkali-free refractory oxide layer of Al.sub.2 O.sub.3, TiO.sub.2, SiO.sub.2, or appropriate mixtures thereof using a lacquer vehicle. U.S. Pat. No. 3,533,827 to Rimbach discloses depositing an inorganic binder of Al.sub.2 O.sub.3 and B.sub.2 O.sub.3 on a lamp envelope as a liquid coating for adhering a phosphor powder thereto. U.S. Pat. No. 3,586,898 to Speros et al discloses a metal halide high intensity arc discharge lamp utilizing AlCl.sub.3 along with mercury to provide a continuous emission spectrum. Speros discloses the need for a high density Al.sub.2 O.sub.3 envelope or an Al.sub.2 O.sub.3 coated fused silica envelope to prevent the reaction of the AlCl.sub.3 inside the lamp with the SiO.sub.2 of the fused silica envelope.